1. Field of the Invention
The present invention relates to continuous measurement of the quantity of coal in a ball grinder.
2. Description of the Prior Art
When using a ball grinder, it is necessary to verify continuously that the quantity of coal is constant, to secure optimum grinding and optimum drying. If the quantity of coal introduced is too great, the grinding is insufficient and the drying is imperfect; if the quantity of coal introduced is too small, the downstream boiler receives insufficient feed.
Systems have been designed to estimate the quantity of coal contained in an operating ball grinder.
A first system is based on the variation in the measured power absorbed by the electric motor driving the ball grinder. This method is of low sensitivity; also, it requires frequent recalibration as the balls wear down or new balls are added.
Another system is based on the use of level sensors.
Pneumatic sensors each including a pneumatic hose with one end inside the grinder are used to measure the pressure difference between two levels; the quantity of coal in the grinder can be deduced from this measurement. However, the level sensors are installed in a hostile environment (coal dust, dropping balls, risk of clogging, etc) with the result that there is a high risk of failure; the sensors are connected to a complex and therefore costly pneumatic unit which is also costly to maintain. The availability of a system of this kind is only moderate.
A further system is based on measuring the noise emitted by the grinder. This method has the drawback of supplying a signal that is highly dependent on the throughput of the grinder, the size of the coal fragments introduced, the quantity of balls present in the grinder and the wear of the armor plating on the inside walls of the grinder.
One object of the present invention is to provide a device for measuring the quantity of coal in a grinder that is accurate, that does not require any equipment to be installed in the hostile environment inside the grinder, and the result of which does not depend on the particle size of the raw coal feed, the throughput of the grinder, the quantity of balls or the degree of wear of the balls.
Another object of the invention is to provide a low-cost high-availability measuring device.
A further object of the invention is to provide a device which can be recalibrated without intervention inside the grinder and without stopping operation of the grinder.